Wind power is considered one of the cleanest, most environment friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor comprising one or more rotor blades. The rotor blades capture kinetic energy of wind using known foil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the rotational energy (mechanical energy) to electrical energy that may be deployed to a utility grid.
During operation of the wind turbine, rotation of the rotor blades through air generates aerodynamic noise. Typically, the aerodynamic noise is driven by three factors including rotor design, rotor speed, and an angle of attack distribution of flow over the rotor of the wind turbine. The angle of attack distribution of flow may change as the rotor turns, for example under conditions of high wind shear. In a part of a rotor plane that is impacted by high wind speed, the angle of attack is high, and thus the noise may be high. A high variation in the angle of attack may cause a variation in aerodynamic noise level, and result in amplitude modulation of the aerodynamic noise.
Due to the amplitude modulation of the aerodynamic noise, a “swooshing” or periodic pulsing sound is typically heard in near fields/habitation of the wind turbine (i.e., the area directly around the wind turbine). Such sounds are typically seen as a nuisance and, thus, regulations are typically put in place establishing maximum sound levels, measured in decibel (dB) for wind turbines operating around residential communities and other populated areas. As a result, wind turbines and wind parks are typically designed to operate below these maximum sound levels. Recently the regulations are extended to include a limit on the amplitude modulation.
Current research suggests that the peak-to-peak amplitude of the modulated noise generated by wind turbines may be higher at locations in far field/habitation (i.e. locations at a certain distance (e.g., 1-4 kilometers) away from the wind turbines) than in the near field due to propagation and/or directivity effects. Therefore, there is a risk that the wind turbines operating below the amplitude modulation limit in the near field may actually be exceeding this limit in the far field.
Various methods have been proposed for reducing the noise emissions of wind turbines. For example, it has been proposed to reduce aerodynamic noise by de-rating all of the wind turbines within a wind turbine farm in order to keep the wind turbines speeds low during time intervals (e.g., during nighttime or other times at which reduced noise is desired). However, such techniques do not reduce the amplitude modulation, and may reduce power produced by the de-rated wind turbines. Accordingly, a system and method for controlling the amplitude modulation of noise generated by wind turbines is desirable.